Utility of a multi-faceted approach in determining the habitat use of endangered euryhaline elasmobranchs in a remote region of northern Australia

Whitty, Jeff
(2011)
Utility of a multi-faceted approach in determining the habitat use of endangered euryhaline elasmobranchs in a remote region of northern Australia.
Masters by Research thesis, Murdoch University.

Abstract

The overriding aim of this thesis was to explore the habitat use of the critically endangered freshwater sawfish (Pristis microdon) and northern river shark (Glyphis garricki) in the remote Kimberley region of northern Western Australia. This information has been largely lacking and is critical for the management and conservation of these species. Habitat use of these species was documented through the use of long-term catch, environmental and tagging (conventional, acoustic and satellite) data, which was acquired between 2005 and 2009 in the Fitzroy River and King Sound, Western Australia. Monitoring of the various environmental parameters demonstrated that the study area is extremely dynamic, with significant seasonal changes in abiotic variables such as water flow, depth, temperature, turbidity and salinity. Catch data demonstrated that juvenile P. microdon inhabit the Fitzroy River for three to five years, at which time individuals begin to emigrate from the river, prior to maturation. Catch data also demonstrated that juvenile and adult male G. garricki inhabit highly turbid nearshore waters throughout the upper King Sound. Relative abundance of P. microdon in the river varied seasonally and annually, and differed between size classes. A decrease in relative abundance between the early and late dry season, which was only significant with young of the year P. microdon, was attributed to mortality as well as dispersal of individuals. Catches of G. garricki were rare, although this species was relatively abundant in highly turbid waters in King Sound. Foraging, depth use and inter-pool movements of P. microdon in the Fitzroy River were influenced by depth, flow and light intensity/turbidity and potentially by salinity and/or water temperature. However, the response to environmental variables differed between P. microdon size classes, possibly due to differences in trophic and habitat requirements of the various size classes. Results from this study demonstrated that P. microdon is sensitive to environmental change, but appear to endure/recover from short-term (months) negative impacts through behavioural regulation. It is not possible at this time to positively conclude about the impacts of specific environmental changes on G. garricki habitat use, due to the limited data from G. garricki captures and tagging. However, as all G. garricki captured in this study were observed to inhabit tidal creeks and river mouth areas, the destruction of such areas or restriction to and from such areas is likely to negatively impact this species.